1. Technical Field
This invention relates generally to manufacturing and shaping of parts constituted of reasonably rigid material, and more particularly to rapid prototyping of such parts by sectioned object machining which facilitates free-form fabrication.
2. Discussion of the Prior Art
It is becoming paramount that prototype parts and their associated tools be fabricated faster in free-form, at lower cost, particularly in metal shapes, have superior surface finish and require little or no remachining. Technology attempting to meet these objectives fall into generally four categories: extruding, spraying, curing and laminate forming. Each has significant disadvantages that fail to meet all of the objectives.
Extrusion uses a heated nozzle to extrude hot wax or other polymer filaments, such as nylon, into a shape. It cannot be used to make metal parts; it is fragile, subject to delamination, and is expensive.
Spraying can be either (i) an emulsion type where a binder is sprayed onto a metal powder which is subsequently laser sintered, or (ii) thermally sprayed at high temperature using a metal powder to produce a shape. Spraying is expensive, slow and produces a mediocre surface finish which is porous.
Curing, often times referred to as stereolithography, involves subjecting photo-sensitive polymer resins in thin layers to a laser beam for curing and thence repeating these steps to build up a part. It is generally slow, useful only for nonmetals and tooling, and requires expensive emission control measures because of the odorous materials. It also demands post machining because the surface is not stable during storage, is somewhat brittle, and is size limited.
Prior art laminate forming involves cutting of metal plys, paper, plastic sheets or foam, by use of a laser beam, hot wire or mechanical cutter to make sheet-like cutouts; the cutouts are subsequently assembled to form a unit. Laminate forming first was carried out without a computer graphic guide, the cutter following a mechanical guide. More recently, the prior art has programmed the cutter to follow a computer model or graphic such as shown in U.S. Pat. No. 4,675,825 patented on Jun. 23, 1987 to DeMenthan. The latter type of laminate forming has been used to shape metal, but fails to eliminate post-machining of the unit, lacks high structural integrity because of the need to use very thin laminates, is incapable of making complex parts such as intricate engine cylinder heads, manifolds or engine blocks, and is slow due to the high number of handling and finishing operations resulting from the number of laminates.
The requirement of post-machining for state of the art laminate forming is a significant drawback. Machining used for this process works only to cut edges of the thin sheet metal (up to 12 mm) inhibiting full contour machining. When the sheets are assembled, the total surface will be somewhat ragged or stepped, requiring post-machining to obtain acceptable surface finishing. If the laminates are foam, paper or plastic, the same problem remains because of the inability to fully free-form the edges of the material; moreover, foam materials can produce a high degree of porosity in the final surface when assembled.